1. Field of the Invention
The invention relates to an adjustable-length gas spring comprising a housing, which has a central longitudinal axis and is filled with a free-flowing fluid; a guide and seal unit, which closes a first end of the housing; a piston rod, which has an outer end and is extended from, and sealed towards, the first end of the housing by the guide and seal unit; a piston, which is connected with the piston rod and sealingly guided in the housing; a first partial volume, which is formed between the piston and the guide and seal unit; a second partial volume, which is defined by the piston and turned away from the first partial volume; and a valve for interconnection of the partial volumes, which has a valve pin which can be actuated from outside the housing and which, when pushed in a valve opening direction in the sense of the central longitudinal axis, is moved into an opening position of the valve and into a closing position by pressure of the fluid counter to the valve opening direction.
2. Background Art
Great numbers of gas springs of the generic type have been described. For instance, so-called double-tube gas springs are known, in which two tubes are arranged concentrically one within the other, defining an annular channel between them. Partial volumes are formed on both sides of the piston, which are interconnected by way of the annular channel and the valve that is disposed on one end of the housing. Adjustable-length gas spring of this type have been known for example from U.S. Pat. No. 3,656,593.
Furthermore, adjustable-length gas springs have been described, in which the valve is disposed in the piston and which are operated by a valve-actuating rod that is disposed inside the hollow piston rod. Gas springs of this type are known for instance from U.S. Pat. No. 4,949,941. In gas springs of that type, hydraulic fluid may fill the partial volumes of the housing; the pressure gas filling is then only located in a pressure gas chamber which is allocated to the closed end of the housing and which is divided from the neighboring partial volume by a slide piston which is guided on, and sealed towards, the inside wall of the housing.
All these familiar gas springs have in common that damages to, or destruction of, the gas spring may occur in the case of excess pressure between the housing and the piston rod i.e., when a force that exceeds given reasonable dimensions tends to pushing the piston rod into the housing. This problem is rather pronounced in the above-mentioned hydraulically lockable single-tube gas springs, for example when used in hospital beds.
EP 1 101 972 A2 teaches an extension gas spring which has a housing with a piston rod concentrically guided therein. Arranged on the piston rod is a piston which is sealingly guided in the housing, dividing an oil chamber into two partial volumes. A valve is available, interconnecting the two partial volumes and having a valve pin, which is actuated from outside the housing and which, when pushed in a valve opening direction in the sense of a central longitudinal axis, is moved into a position in which the valve opens and, by the action of a restoring spring counter to the valve opening direction, into a position in which the valve closes. Integrated into the housing subsequently to the oil chamber is an extension gas spring which the piston rod is led into.
Inside the piston that divides the oil chamber into partial volumes, provision is additionally made for a pressure relief valve of the type of a check valve which includes a channel that passes through the piston and interconnects the partial volumes and that is closed on one side by a spring-loaded packing disk. This pressure relief valve opens when a force that exceeds an admissible force acts between the piston rod and the housing.
It is an object of the invention, to embody a gas spring of the generic type such that damages to, or destruction of, the gas spring are avoided even in the case of high forces of pressure that exceed given admissible thresholds.
According to the invention, this object is attained in a gas spring of the generic type by the features which consist in that, when a force that acts between the piston rod and the housing in a push-in direction of the piston rod exceeds a given threshold, the valve pin is displaceable from the closing position counter to the valve opening direction and against a spring unit into a position of opening by overload. The gist of the invention resides in that the valve pin, upon excess pressure, may deviate counter to the direction of opening, releasing a junction between the partial volumes for pressure balance to take place within the gas spring.
Further features, advantages and details of the invention will become apparent from the ensuing description of an exemplary embodiment, taken in conjunction with the drawing.